A solid oxide fuel cell (SOFC) using a solid oxide, i.e., a ceramic material, has high efficiency compared with other fuel cells, and since it can use various types of fuel besides hydrogen, the SOFC has been developed mainly for the purpose of large scale power generation. Recently, as demand for mobile power having high output and high energy density is on the rise, development of a small SOFC as a small mobile power source is paid much attention.
In order to enhance cost-effectiveness of an SOFC for the purpose of large scale power generation and implement a small SOFC, it is required to lower a current operational temperature ranging from 800° C. to 1000° C. The high operational temperature causes an interface reaction, degrades performance due to thermal expansion mismatching between constituents such as electrolyte, electrodes, a sealing material, and the like, and limits materials and components which may be used, degrading economic feasibility. In particular, when the SOFC is applied as a mobile power source, lowering of the operational temperature is crucial. In this respect, however, the lowering of the operational temperature lowers conductivity of electrolyte or activity of a catalyst to result in a reduction in performance, so, in order to cancel it out, a new material is required to be employed or the structure is required to be changed.
In particular, compensation for the reduction in the conductivity of electrolyte according to the lowering of the operational temperature by lowering resistance by reducing the thickness of electrolyte is one of important research fields. In case of the most commonly used electrolyte, yttria stabilized zirconia (YSZ), when it has the current thickness of a few microns, it can have satisfactory performance at an operational temperature of about 700° C. or higher, but when the thickness of the YSZ is reduced to below 1 micron, YSZ can be operable even at 500° C. or lower.
In order to use an electrolyte thin film layer, generally, the electrolyte thin film is deposited on a dense substrate such as a silicon wafer, and then, a free-standing membrane is generated by using an MEMS process to implement a fuel cell (U.S. Pat. No. 6,638,654 B2, US 2007-0184322A, etc). However, such a membrane type fuel cell is structurally very vulnerable, and in particular, it cannot avoid thermal, mechanical vulnerability at an operational temperature of hundreds of degrees Celsius and heat cycle. Thus, it is advantageous to implement an electrolyte thin film layer on a support having a porous structure in terms of thermal, mechanical stability and long-term stability.
However, when an electrolyte layer is formed on the porous substrate, when the thickness of the thin film is smaller than the size of pores, a defect is generated due to the pores (DeJonghe et al., Annu. Rev. Mater. Res. 2003. 33:169-82). Thus, it is not possible, in actuality, to form a dense electrolyte layer having a thickness of 1 micron or smaller on the existing porous substrate having the size of pores of a few microns. When a general anode support is used as a substrate, starting powder has a size of microns, and as a sintering temperature reaches about 1000° C., the size of particles has a few microns. In this case, a surface roughness and the size of pores have a few microns, so in order to cover them with a completely dense membrane without having gas permeability, the thickness of the electrolyte layer is required to correspond to the size of the pores or greater.
Thus, in order to use a thin-film electrolyte, controlling of the pore structure at the support part in contact with the electrolyte layer is requisite. Here, in order to resolve this, if the entirety of the support is implemented to have a structure having a particle size or a pore size of micron or lower, there is much restriction in terms of process to control a microstructure, causing very complicated processes and an instruction of gas is hampered to degrade the performance of the SOFC.
Also, although the pore structure and surface roughness of the porous electrode substrate are controlled, if the difference in sintering shrinkage between the substrate and the electrolyte layer and a defect such as disintegration of the electrolyte layer due to a grain growth are not restrained in the electrolyte layer formation process, the occurrence of a fatal defect in the electrolyte layer cannot be avoided. Since shrinkage generated in the electrolyte layer in a drying and sintering process, or the like, is greater than that of the rigid substrate, tensile stress is generated in the electrolyte layer, inevitably causing a defect.
A method for implementing the porous support having a gradient structure is also disclosed in U.S. Pat. Nos. 5,114,803 and 6,228,521. 5,114,803 particularly relates to a method for forming a gradient structure to improve diffusion and a three-phase boundary at an electrode support in a cathode-supported tubular SOFC. In this patent, a general powder process and a high sintering temperature (>1300° C.) are used, so although a gradient structure is formed, it is impossible to form a dense electrolyte layer within 1 micron at an upper portion of a porous electrode. Also, in the embodiment of the patent, the thickness of electrolyte is 100 microns. U.S. Pat. No. 6,228,521 presents a gradient structure having a lower portion containing a large amount of nickel (Ni) compared with a general composition and an upper portion containing a smaller amount of nickel (Ni) in order to obtain high porosity at a lower portion of an Ni-YSZ porous substrate and enlarge the three-phase boundary at a portion in contact with electrolyte. In this patent, however, the gradient structure is formed at high temperature through the existing powder process, and surface roughness, pore size, a pore size distribution, and the like, are not adjusted, so a thin film electrolyte layer cannot be formed at an upper portion.